Heavy crude oil is closely related to natural bitumen from oil sands with respect to a number of properties. Generally, bitumen is the heaviest, most viscous form of petroleum and is often referred to as “natural bitumen.” Bitumen shares the attributes of heavy oil but is more dense and viscous. Natural bitumen and heavy oil differ from light oils by having higher viscosity (resistance to flow) at reservoir temperatures. As is known, heavy oil is often found at the margins of geologic basins and is thought to be the residue of formerly light oil that has lost its light-molecular-weight components. Conventional heavy oil and bitumen differ in the degree by which they have been degraded from the original crude oil. Often, bitumen does not flow under ambient conditions within a given reservoir.
The large reserves of bitumen and heavy oil in the Alberta oil sands have been under development for many years and the pace of development is accelerating. While certain areas of the oil sands are being developed by strip-mining due to the proximity of the bitumen to the surface, many other areas where the bitumen is well below the surface are being developed using advanced processes which have a significantly lower impact on the landscape. One well known process is steam-assisted gravity drainage (SAGD) which typically utilizes two or more vertically displaced horizontal wells and high pressure steam that is continuously injected into an upper wellbore to heat the reservoir. As a result, the viscosity of the heavy oil/bitumen within the reservoir is reduced, thereby enabling it to flow downward to a production well. While effective, SAGD is energy intensive and requires significant surface infrastructure to manage the steam production and water/oil recovery and separation.
Another process for recovery of heavy oil and bitumen has been developed by the present applicant. This process, known as thermally-assisted gravity drainage (TAGD) has been described in US Patent Publication No. 20120318512 which is incorporated herein by reference. In TAGD, also using horizontal wells, the mobility of the bitumen or heavy oil is increased by conductive heating (instead of steam) to reduce its viscosity. In these processes, the bitumen or heavy oil is heated to temperatures below the thermal cracking temperature of the bitumen or heavy oil. As the bitumen or heavy oil is produced, evolved gases, evaporated connate water or both form a gas chamber which acts to replace the volume of the produced fluid required for the gravity drainage process. Some of the more common applications of this process use heaters placed in wells drilled in specific patterns surrounding the main producer well. The patterns have been developed by extensive reservoir modeling studies for optimizing placement of heaters for optimal conduction of heat within the reservoir. These heaters are hereinafter referred to as “well heaters.” TAGD provides a number of advantages over SAGD processes including reduced energy and surface infrastructure costs.
A number of other processes for recovery of heavy or bitumen are under development which will also require the use of well heaters. A number of different types of heating means may be provided in well heaters used for TAGD or other similar processes. Examples of such heating means may include dielectric heating (also known as electronic heating, RF heating and high frequency heating), hot water circulating heaters, catalytic heaters, fluid exchange heating, and heating using molten salts or metals. One particularly useful class of well heating mechanism is resistance heating (also known as Joule heating and Ohmic heating). This heating mechanism is typically provided using cables with resistive portions that release heat when subjected to electric currents. The heater cables are typically run into wells using coiled tubing.
Because processes such as TAGD require heating of deep reservoirs, the lengths of the well heater cables and their protective components which make up the body of the heater (hereinafter referred to as well heaters) may be several thousand meters in length. A number of problems are associated with assembly of such well heaters.
In the past, well heaters with resistive cables were assembled in areas with very long sections of clear flat ground, such as unused aircraft runways. Typically, a long section of coiled tubing would be unwound onto the runway and secured to the ground using large heavy weights to maintain the straightness of the coiled tubing. The heater cables would then be pulled into the coiled tubing by inserting a tow cable through the coiled tubing and then pulling the heater cable through it. After the components were assembled, the assembled heater would be spooled onto a standard coiled tubing reel and then transferred to the wells for deployment. Not surprisingly, this method of assembling well heaters has significant drawbacks. For example, assembling a heater cable on a disused runway has significant risks, including the risk of contamination and/or damage to the cables as a result of dragging them over ground or pavement, safety risks associated with handling large weights to safely secure the coiled tubing in a straight line, as well as the practical limitation of identifying the required stretches of clear flat ground or pavement. This method is also labor-intensive and would typically require on the order of 25 workers about 6 days to assemble a single well heater. Furthermore, this assembly method is also affected by the prevailing weather conditions.
Accordingly, there has been a need for improved systems and methods of assembling heater cable systems and, in particular a need for systems that overcome the problems of assembling well heaters in an uncontrolled outdoor environment.
More specifically, there has been a need for systems that enable the controlled “indoor” assembly of well heaters. In addition, there has been a need for improved well heaters that can be readily assembled to a desired length with specific properties.
A review of the prior art indicates that various heater systems have been developed relating to various components of the heater systems and the equipment required for handling and deployment of heater systems and coiled tubing. For example, the construction of a “temperature limited” well heater is described in U.S. Pat. No. 8,579,031.
A gripper block for a coiled tubing injector with a variable tubing size capability is described in U.S. Pat. No. 6,892,810.
US Patent Publication No. 2010/0224368 describes a method for making a coiled insulated conductor heater to heat a subsurface formation. The method described in this reference includes the step of pushing the insulated conductor heater longitudinally inside a flexible conduit using pressure, wherein one or more cups are coupled to the outside of the insulated conductor heater. The cups are configured to maintain at least some pressure inside at least a portion of the flexible conduit as the insulated conductor heater is pushed inside the flexible conduit.
US Patent Publication No. 2010/0089584 describes a heater for treating subsurface formations which includes a conduit and three insulated electrical conductors located in the conduit.
US Patent Publication No. 2013/0086800 describes a process for forming insulated conductor heaters using a powder as the insulator. The process includes steps of feeding of sheath material such as stainless steel and conductor (core) material into a process flow line and passing these components through compression and centralizing rolls to form tubular materials, followed by addition of heated electrical insulator powder into the sheath.
U.S. Pat. No. 8,502,120 describes an insulated conductor heater with an electrical conductor that produces heat when an electrical current is provided to the electrical conductor. An electrical insulator at least partially surrounds the electrical conductor. The electrical insulator comprises a resistivity that remains substantially constant, or increases, over time when the electrical conductor produces heat.
US Patent Publication No. 2013/0118746 describes a system for use in an in situ oil production process which includes a multi-component composite cable having multiple conductors for delivering electrical power to a heater array, multiple hoses for transmitting fluid to a heater array, a strength member made of a heat resistant synthetic fiber material, and a cable jacket layer surrounding the conductors, hoses and strength member.
U.S. Pat. No. 4,570,715 describes an electrical heater containing spoolable, steel sheathed, mineral insulated cables which have high electrical conductivities. The conductors are surrounded by heat stable electrical insulations such as a mass of compacted powdered mineral particles and/or by discs of ceramic materials.
In view of the foregoing, there continues to be a need for improved well heater systems and processes for assembly of these well heaters.